1. Field of the Invention
The invention relates to ring lasers and more particularly to internally biasing ring lasers.
2. Description of the Prior Art.
A ring laser essentially consists of a polygonal array of mirrors, which are arranged to enclose an area and form a resonant cavity which includes a gain medium. This resonant cavity has two oppositely traveling resonant light fields that traverse the same path and oscillate at frequencies which are determined by the optical path lengths they travel. Effects which are measured using ring lasers cause differences in the optical path lengths of the oppositely traveling beams which results in a frequency difference between the beams. By monitoring this frequency difference the magnitude and direction of the effect are determined. A number of applications for which ring lasers have been used are: as gyros having no moving parts and unaffected by acceleration or orientation; as highly sensitive and accurate fluid flow meters; and in devices for studying changes in the refractive index of materials.
Heretofore ring lasers have experienced beam coupling when the difference in beam frequencies was below a certain lock-in frequency. At frequency differences below the lock-in level the frequencies would spontaneously coincide. This was caused principally by energy exchange between light in one beam and back scattered light from the other beam, and also by competition effects within the lasing medium. These effects precluded the use of ring lasers at frequency differences below the lock-in frequency.
Energy exchange is generally considered to be caused by back scattered light from one beam reinforcing the other beam when the frequencies of the two beams are nearly equal. See M. Bass, H. Stotz, and G. A. DeMars, J. Appld. Phys., vol. 39, p. 4015-4018, July, 1968. Practical limitations seem to prevent total elimination of back scatter; therefore, in the prior art a frequency bias was used to separate the frequencies of the two beams to prevent beam coupling. The use of a frequency bias also reduced mode competition effects within the laser medium.
Various bias techniques have been used with varying success to overcome this beam coupling phenomenon. See J. Kilpatrick, IEEE Spectrum, vol. 4, p. 44-55, Oct. 1967. Some of the prior biasing techniques utilized an elasto-optic diffraction element as part of the ring laser cavity. These elements were used to either amplitude modulate the intensity of the oppositely traveling beams or to produce a frequency difference between the beams by Brillouin scattering.